Process for recovering isobutylene from acid mixtures



United States Patent 2,992,284 PROCESS FOR RECOVERING ISOBUTYLENE FROM ACID MIXTURES Robert A. Sanford and Robert L. Foster, Homewood, Ill., assignors to Sinclair Refining Company, New York, N.Y., a corporation of Maine No Drawing. Filed Apr. 7, 1960, Ser. No. 20,526 7 Claims. (Cl. 260-677) This invention relates to an improved process for the removal of iso-olefins from fat mineral acids containing the same and more particularly to an improved process for the recovery of isobutylene from a fat mineral acid by contacting such fat acid with a liquid hydrocarbon under certain conditions.

It is known in the prior art that sulfuric acid will selectively remove isobutylene from hydrocarbon streams containing the same and other C hydrocarbons primarily the butanes and n-butenes. Such a hydrocarbon stream is generally obtained as a side out in the distillation of the product resulting from the catalytic cracking of petroleum gas oils and will usually be composed of about 10 to 25% of isobutylene. A typical butane-butylene stream composition can be represented as follows:

Percent Isobutylene 14 Butanes 54 n-C C 2 C 2 The conditions for such an extractive procedure are chosen so that the greatest amount of isobutylene will be removed from the stream with the least amount of polymerization, for instance, sulfuric acid concentrations of about 50 to 75% by weight, preferably about 6065-% by weight; temperatures in the range of about 0 to 50 C., preferably about to 20 C.; and periods of time to allow for the attainment of equilibrium between the hydrocarbon phase and the 'acid phase. The pressure is sutficient to maintain the liquid phase; The hydrocarbon stream to acid ratio will generally be in the range of about 2 to 7:1. By following such an extractive procedure a sulfuric acid extract will be obtained having about 15 to 50% isobutylene absorbed therein. In order to recover the isobutylene from the fat acid extract it has been the customary practice to dilute the acid with water to about 40 to 45% by weight concentration and thereafter distill. This dilution of the acid although allowing for the distillation of the isobutylene without extensive polymerization introduces the undesirable problem of acid reconcentration prior to recycling it back to the acid extraction unit.

By the present invention we have overcome this prior disadvantage of acid reconcentration. In proceeding in accordance with the present invention, a fat sulfuric acid which can be obtained essentially as outlined above is combined in a hydrocarbon contacting zone for a short period of time with a contacting liquid hydrocarbon maintained at a temperature substantially above the boiling point of the isobutylene and in a state of ebullition during isobutylene separation. The time of contact will be primarily dependent upon the temperature of the liquid hydrocarbon so that undue polymerization Will be avoided. For example, contact times in the range of about 30 seconds up to about 5 minutes and preferably, slightly over one minute, e.g. about 65 to 100 seconds, will yield good isobutylene recovery if the contacting liquid hydrocarbon is maintained at about 50 to 125 C. and preferably about 70 to 100 C. Not only must these time and temperature conditions be observed but also we have found it necessary that the contacting liquid 2,992,284 Patented July 11, 1961 hydrocarbon be in a state of ebullition during isobutylene separation. By the use of th term ebullition we intend to mean the passage of a stripping fluid through the liquid hydrocarbon so that a bubbling or boiling effect is obtained.

In the present invention we have found that this ebullition of the liquid hydrocarbon can be most advantageously attained by either maintaining the liquid hydrocarbon at boiling temperature so that the hydrocarbon itself acts as the stripping fluid or by maintaining the liquid hydrocarbon at less than its boiling temperature and passing a non-aqueous inert gas such as nitrogen therethrough. Thus, the temperature at the top of the liquid hydrocarbon Zone is at least the effective boiling point of the contacting hydrocarbon or an extraneous non-condensing sweep gas must be employed. This gas should be essentially inert to the hydrocarbons present under the conditions of contact. The isobutylene thus contacted is removed from the acid by distillation without undergoing undue polymerization and the lean acid passing from the bottom of the zone contains substantially no isobutylene and can be recycled directly back to the acid extraction without an additional acid reconcentration step. Even through this recycled lean acid does contain some i-C there will be no undue polymerization thereof since it will go directly back to the acid extraction unit with no deleterious change in temperature. Although it is preferred to separate the isobutylene from the mixture of contacting hydrocarbon and fat acid in the vessel of mixing We also contemplate the procedure in which these operations are conducted in distinct vessels with intermediate separation to remove the acid phase. We particularly prefer, however, that a single vessel be used and that the fat acid enter the liquid contacting hydrocarbon zone in the upper part of the body of liquid. The isobutylene distilled can be passed through condensers to separate any entrained hydrocarbon and then collected. The entrained hydrocarbon removed by the condenser can be returned to the main body of liquid hydrocarbon thereby reducing the need to replenish the supply of liquid hydrocarbon.

We are aware of U.S. Patent No. 2,443,245 to Hibshman in whose process isobutylene is removed from a sulfuric acid extract by countercurrent contact with a vaporized narrow boiling hydrocarbon mixture having a boiling range of about 30 to 40 F., the lower boiling components of the mixture boil at about the same temperature or above that of the highest temperature used in the stripping column and the higher boiling components boil below 350 F. and above 200 F. This system has proven to be difiicult to control due to the fact that the temperature is dependent on the vaporized hydrocarbon/ acid ratio, any change in this ratio resulting in a temperature change which can readily lead to additional polymerization of the isobutylene.

By the use of a hydrocarbon in the liquid phase as the contacting medium rather than in the vapor phase as in the Hibshman patent, we are able to maintain indi- .vidual control over the variables that would tend to lead to additional polymerization. For example, our contacting temperature is maintained at about 50 to 125 C., preferably 70 to 100 C., quite independently of the hydrocarbon/ acid ratio which can be varied from about 20 to 1 up to about 200 to 1 or more, preferably greater than to 1, with no excessive polymerization. By maintaining individual control over the distillation conditions we alford a product of increased purity and lower formation of polymer than does the process of the above patent.

The contacting hydrocarbons which we have found most advantageous for use in the present invention are those which are a liquid under normal conditions, are inert to the acid and to the isobutylene, and have a boiling point high enough to be readily separated from the fective recovery of the isobutylene from the fat acid several tests were made. Since the general conditions for extracting the isobutylenes from a butane-butylene stream with sulfuric acid are well known, all the fat acids used in the following runs were simply prepared by passing the desired percentages, usually about to 50% of isobutylene directly into the sulfuric acid at l0 C.

with stirring. In all the runs, except 8l2-35, 8l2-37A,

and 81237B the fat acid-hydrocarbon contact was effected in a three-necked separatory flask fitted with a condenser and stirrer. The hydrocarbon was placed in the flask and preheated to the desired temperature. A short prerun was usually provided to establish equilibrium conditions. The fat acid was added to the upper portion 'of the liquid contacting hydrocarbon layer (when present) and the lean acid withdrawn continuously as bottoms.

The isobutylene which flashed off was passed through the condenser to separate entrained hydrocarbon contacting liquid. The hydrocarbon thus separated was-returned directly back to the flask as reflux. The. contact time was determined in each case from the rate of addition of the 'fat acid and the amount of holdup in the flask at the end of each run. The contact time refers to the time of contact between the hydrocarbon liquid, and each increment of fat acid, and the weight of the fat acid given in the tables below is the total weight of fat acid added incremently while the hydrocarbon/acid ratio is the amount of hydrocarbon contacted with each increment of fat acid. The above excepted runs were performed in a column packed with glass beads.

The following comparative data will serve to illustrate the present invention and the most favorable conditions for carrying out the invention.

In order to illustrate the necessity of the presence of the contacting hydrocarbon in such procedures, two separation tests were conducted. The conditions utilized and the results are reported in Table I below.

TABLE I Fat acid: Percent H1304 W Nitrogen added Percent isobutylene moval Percent isobutylene recovered based on feed... 0

393.5 gms. of 65% H1804 were present in the flask.

In run 78891 it will be noted that no liquid hydrocarbon was present nor was the mixture in a state of ebullition. Further, it, was found that polymer was formed in an amount equal to the isobutylene in the fat acid. In run 812-35 no liquid hydrocarbon was present; however, ebu-llition of the mixture was eflected by passing nitrogen therethrough. In neither of the above runs was a substantial amount of isobutylene recovered indicating that it is not the heat nor the-ebullition alone which produces the isobutylene recoveries of the present invention.

The following runs indicate the advantages obtainable by the use of a liquid hydrocarbon stripping agent rather '4 than a vaporous stripping agent. The results of these runs are tabulated in Table II below.

TABLE II Fat acid:

Weight (g.) 108.5 108. 5

Percent i-Ci= 25. 7 25. 7 Contacting hydrocarbo Cyclohexane weight (g.) 1 173 31 Conditions:

Temperature, C.) 8288 84-87 Contact time, (sec.) 30 45 i-O4=Rem0val from [at acid (percent) 100 98. 7 i-C Recovery (percent) based on feed:

Traps (Dry Ice) 7. 2 44. 4

Dissolved in cyclohexane 20. 5 5. 5

1 Vapor.

In run 812-37A above the column contained cyclohexane vapor with no condensation taking place whereas in run 812-37B the contact zone was only partially filled with vapor, i.e. the main body of the hydrocarbon was in the liquid phase due to refluxing, the vapors providing the sweep gas. Obviously, the presence of the extracting hydrocarbon in the liquid phase greatly enhanced the i-C recovery.

Thus it can be seen from an examination of Tables I and II the presence of the hydrocarbon is essential to good isobutylene recoveries and more particularly it is the presence of a liquid hydrocarbon which along with ebullition produces the improved yields of the present invention.

The most advantageous time for the liquid hydrocarbon-fat acid contact to obtain the highest yields of isobutylene and the lowest amount of polymerization was studied and the data is reported in Table III following.

TABLE III Fat Acid (g.) 11 123. 5 118 121. 5 116. 5 Percent 1-0 20. 4 21. 5 21. 7 20. 4 26. 6 Strength of H180 percent 65 65 55 65 65 Cyclohcxane (g.) 200 204. 6 202 1 200 201 Contact time (so 20 26 48 70 900 Temperature G.) 73-74 73-76 70-72 74-75 Percent x-O4= Removed from acid 1 53. 5 62. 2 70. 3 86. 6 Percent 1-C4= Recovered 53. 51.0 76. 2 80. 6 53. 2

1 257 ml. 9 From weight loss of the acid. 3 Based on feed.

An examination of the above table will indicate that the contact time must generally be in the range of about 30 seconds to 5 minutes depending, of course, on the temperature- Preferably the contact time should be slightly in excess of one minute, e.g. about 65 to 100 seconds, at a temperature of 75 C. when using cyclohexane as a solvent. The extremely long contact time of run 812-18, although giving complete removal of the isobutylene from the acid, caused a high rate of polymerization resulting in poor recovery of isobutylene. Contact times substantially below about 30 seconds resulted in poor separation of isobutylene from the acid extract which necessarily gives poor ultimate recovery. do run $12-12, for instance, the fat acid was added to the upper portion of the liquid contacting hydrocarbon layer at a feed rate of 1.02 ml. per minute and the lean acid was withdrawn at a rate of 0.84 ml. per minute. Since about 257 ml. of cyclohexane is present in the flask and the contact time of each increment (e.g. 1.02 ml.) of fat acid is about 70 seconds, the hydrocarbon to acid ratio is about 200 to 1.

At substantially the same temperature the recovery of isobutylene from the fat acid is lower when using nonrefluxing higher boiling hydrocarbons rather than refluxing cyclohexane, further indicating that it is not merely the heat and the liquid hydrocarbon which brings about the removal and recovery of the isobutylene from the fat acid, but rather that the system must also be in a state of ebullition. The pertinent data for these runs are in Table IV below.

TABLE EV Efiect of solvent 812-12 ISM-14B 812-13 812-3 Contacting hydrocarbon Cycle-Co n-C1 n- C1 n-Cw Amoun 200.0 210.0 200.0 201.0 Fat acid (H2804):

Weight 121. 5 113.0 123.0 117.8 Concentration of acid (percent) 65 65 65 65 Percent i-C 20. 4 26. 6 20. 4 22. 8 Conditions:

Temperature 0.). 1 74-75 72-76 72-78 80 Contact time (sec.). 70 49 76 18 Percent i-C =Removal from acid 86.6 68. 1 75. 3 66. 5 Percent i'C-l= Recovered based on feed 80.6 6. 7 35. 7 14. 9

I Refluxing.

It was also observed that by increasing the temperature of the higher boiling liquid hydrocarbon up to reflux temperature, the recovery of isobutylene was substantially increased. See Table V.

TABLE V Eflect of reflux It was further observed that an increase of the temperature when using n-decane from 80 C. to 100 C. had no substantial effect on the amount of isobutylene recovered.

It is evident from an examination of the above data that no one of our essential processing conditions taken separately will bring about the improved recovery of iso-butylene but rather that the conditions are interdependent and must be adjusted so as to give a liquid hydrocarbon in contact with the fat acid and ebullition conditions must be eflected for a period of time sufficiently short to avoid polymerization yet adequate to yield substantial amounts of isobutylene. It was further noted that in the experiments using refluxing hydrocarbon small amounts of the contacting liquid hydrocarbon were usually carried overhead. This entrained hydrocarbon can be readily separated by condensers and returned to the main body of refluxing hydrocarbon.

As noted previously the ebullition of the hydrocarbon can also be effected by passing an inert gas such as nitrogen through the liquid hydrocarbon mixture with equally as good results as those obtained by refluxing. By proceeding in this manner, a higher boiling liquid hydrocarbon can be utilized without the necessity of increasing the temperature up to its reflux temperature, thereby lessening the possibility of polymer formation. Furthermore, the use of the higher boiling hydrocarbon and nitrogen ebullition reduces the amount of entrained hydrocarbon carried overhead with the isobutylene. Table VI below shows the isobutylene recoveries obtaina- D ble by bubbling nitrogen through a n-decane-fat acid mixture maintained at a temperature less than the reflux temperature.

TABLE VI Efiect of nitrogen ebullition Solvent (N-Cm): Weight (g.) 201 151. 5 200.0 150.0 Fat Acid (H2804):

Weight 117. 8 103.8 59. 5 113.0

Cone. of acid (percent) 6 65 65 65 i-C4= (percen 22.8 24. 2 22. 8 20. 5 Conditions:

Temperature C.) 80 p 76 Contact time (see) 18 45 89 48 Nitrogen purge N0 N o No Yes Percent i-C4=Removed from acid 66.5 78. 8 84.5 81. 9 Percent i-O4= Recovered 1 14. 9 34.0 0.0 64. 9

1 Based on the feed.

The fact that a substantial amount of the isobutylene was removed from the acid but the percent isobutylene recovered was low in the runs wherein no inert gas was bubbled through the mixture further substantiates the fact that ebullition is necessary to provide good recoveries. The volume of inert gas used in such an operation as this should be suflicient to provide an elfective sweeping operation.

As mentioned above, it is, of course, possible to use acids other than sulfuric for preparing the fat acid. For example, a fat acid was prepared by passing isobutylene into grains of 86.7% phosphoric acid. The fat acid contained 24.8% isobutylene. This fat acid was contacted in the flasic process described above with 200 grams of cyclohexane refluxing at 74 to 76 C. The contact time was 62 seconds. This contact elfected the removal of 90.7% of the isobutylene from the fat acid and 80.5% of the isobutylene was recovered. Generally in extracting hydrocarbon streams with phosphoric acid the concentration of the acid should be from about 75 to 90 percent by weight.

This application is a continuation-in-part of our application Serial No. 701,700, filed December 10, 1957 and now abandoned.

We claim:

1. In a method for recovering isobutylene from admixture with anacid selected from the group consisting of about 50 to 75% sulfuric acid. and about 75 to 90% phosphoric acid, the steps which comprise contacting said mixture with at least about 20 volumes of a liquid al phatic hydrocarbon containing at least six carbon atoms to one volume of acid, and subjecting the resulting hydrocarbon and acid mixture to ebullition at a temperature of about 50 to C. for a period of about 30 seconds up to about 5 minutes and recovering isobutylene as overhead.

2. In a method for recovering isobutylene from admixture with an acid selected from the group consisting of about 50 to 75% sulfuric acid and about 75 to 90% phosphoric acid, the steps which comprise contacting said mixture in a contacting zone with at least about 20 volumes of a liquid aliphatic hydrocarbon containing at least six carbon atoms to one volume of acid, said hydrocarbon being in a state of ebullition at a temperature of about 50 to 125 C. for a period of about 30 seconds up to about 5 minutes and recovering isobutylene as overhead.

3. The method of claim 2 in which about 75 to 200 volumes of liquid aliphatic hydrocarbon are employed.

4. The method of claim 3 in which the acid is sulfuric acid.

5. The method as described in claim 4 wherein ebullition is effected by maintaining the liquid hydrocarbon at its reflux temperature.

6. The method in accordance with claim 4 wherein iebullition is eifected by maintaining the liquid hydroinert gas is nitrogen.

References Cited in the ,file of this patent 7 UNITED STATES PATENTS Scheeline Sept. 10, 1946 Hibshman June 15, 1948 Morrell et a1. July 10, 1951 Francis .-.-4-- 11113 2. .95 

1. IN A METHOD FOR RECOVERING ISOBUTYLENE FROM ADMIXTURE WITH AN ACID SELECTED FROM THE GROUP CONSISTING OF ABOUT 50 TO 75% SULFURIC ACID AND ABOUT 75 TO 90% PHOSPHORIC ACID, THE STEPS WHICH COMPRISES CONTACTING SAID MIXTURE WITH AT LEAST ABOUT 20 VOLUMES OF A LIQUID ALIPHATIC HYDROCARBON CONTAINING AT LEAST SIX CARBON ATOMS TO ONE VOLUME OF ACID, AND SUBJECTING THE RESULTING HYDROOCARBON AND ACID, AND SUBJECTING THE RESULTING HYTURE OF ABOUT 50 TO 125* C. FOR A PERIOD OF ABOUT 30 SECONDS UP TO ABOUT 5 MINUTES AND RECOVERING ISOBUTYLENE AS OVERHEAD. 